1. Field of the Invention
This invention relates generally to the field of optics and more particularly to an instrument for the measurement of colour.
2. Description of the Prior Art
Standards for the measurement of colour have been set by the Commissioner International de l'Eclairage (hereinafter referred to as CIE) and are used universally and reference will be made throughout this disclosure to these CIE standards.
Colour perception is dependent upon the characteristics of each observer's eyes and varies from person to person. A luminosity function which represents the "standard" eye has been formulated by the CIE and is defined as the y function which peaks in the green part of the spectrum [x and z are similarly defined functions peaking in the red and blue parts of the spectrum respectively].
As any particular light source has s specific energy distribution and a coloured object has a particular absorption characteristic, the perceived colour of the object is dependent upon both of these factors.
Quantitative specification of colour may be made by three tristimulus values X, Y Z which are the integrals of the products of the CIE functions x, y, z with the radiant energy distribution function from the object.
Two chromaticity values can be derived from the following relations: EQU x=X/(X+Y+Z) (I) EQU y+Y/(X+Y+Z) (II)
which can be represented on a chromaticity diagram for colour determination.
The measurement of colour has been well documented in the past and many devices have been made to perform this function. The modern devices may be generally divided into two classes:
(A) THOSE USING OPTICAL TRANSMISSION FILTERS, AND
(B) THOSE USING SPECTROPHOTOMETRIC SCANNING AND SUBSEQUENT DIGITAL COMPUTING.
The first class of devices has high light throughput but suffers from the inherent disadvantage that an optical transmission filter cannot be made to accurately conform with the standard distribution functions set by the CIE and therefore, is not standardized.
The latter class of devices have a relatively slow response, low light throughput thereby resulting in a poor signal to noise ratio and they require digitisation of a spectral response curve and subsequent digital integration to determine tristimulus values and then the chromaticity coordinate values.
These devices and their theory of operation have been described in "The Science of Colour" published by the Optical Society of America. Reference is also made to "New Spectrophotometric and Tristimulus Mask Colourimeter" by Kok, C. J. and Boshoff, M. C. published in Applied Optics, December 1971, Volume 10, No. 12 commencing on page 2617 for a description of an apparatus which may be used both as a spectrophotometer and as a tristimulus colour meter.
Other devices related to colour measurement are disclosed in U.S. Pat. No. 3,314,327 granted to Killpatrick et al on Apr. 18, 1967 and assigned to Honeywell, Inc. and U.S. Pat. No. 3,522,739 granted to Con et al on Aug. 4, 1970 and assigned to Princeton Applied Research Corporation. The former patent discloses a device employing spatial filters and three spectra in a colour meter capable of producing an electrical output indicative of the colour of a substance. The latter relates to a spectrophotometer apparatus utilizing a ratio measuring circuit. These devices all provide a method of specifying the colour of a target or sample in three basic units.
However, the disadvantages of the devices of the prior art have been many. The optical filters used in the devices of the prior art are complex and are made to exacting specifications in order to achieve uniformity resuting in great expense, and whilst having uniformity, they can be made only to approximate the CIE standard functions.
Instruments using spectral scanning techniques are in general, large and opto-mechanically complex. They have slow responses due to the need to scan slowly to achieve an adequate signal to noise ratio for the absorbtion characteristics of the sample. Subsequent digitisation and digital computation, involving a digital integration, can result in loss of accuracy in the case of discontinuous or rapidly varying characteristics where information is lost between digitised data points.
The devices of the prior art which use spatial filter techniques have not been developed to the stage that the advantages of fast responses, high accuracy, wide dynamic range, standardisation and simplicity of construction and manufacture have been achieved.